1. Field of the Invention
The present invention is related to the field of light polarization converters, and more specifically to an improved polarization converter that uses beam splitting cubes.
2. Description of the Related Art
Naturally occurring light is not polarized, which is a state also known as unpolarized. Polarized light is desirable for various applications. Polarized light is derived from unpolarized light using a polarizer, as explained below.
Referring now to FIG. 1, a beam 22 of unpolarized light is incident upon polarizer 24, which operates on the light. At least one beam 26 of polarized light exits polarizer 24. The incident beam travels along a propagation direction 28, and the exiting beam travels along a propagation direction 30. In many applications, direction 30 is the same as direction 28.
Referring now to FIG. 2, the polarization of beam 22 is explained in more detail. A point 40 is considered along direction 28. What the human eye perceives as light is really traveling electric field vectors 44, 46. Although only two such vectors are shown, in fact there can be many. For unpolarized light, their points are distributed uniformly around circle 48.
It is conventional to analyze the electric field in terms of its components along two orthogonal axes 52, 54, that are perpendicular to propagation direction 28. It should be noted that the axes 52, 54 can be translated along any point of direction 28, such as point 40. This analysis is useful for discussing polarizers. The directions of axes 52, 54, are also known as S and P directions.
It can be seen, therefore, how a single vector 46 relates to the two axes 52, 54. Each vector would have a component on axis 52, and a component on axis 54. In fact, all vectors with points on circle 48 can be similarly decomposed into components on the axes 52, 54. When they are all so decomposed, all the vectors for light beam 22 are added along each axis. This results in two vectors 72, 74, that represent the whole beam 22, for polarization purposes. Due to the symmetry of circle 48, the two vectors 72, 74 are equal in intensity for unpolarized light.
Referring now to FIG. 3, the action of polarizer 24 of FIG. 1 can be better appreciated. A light at a point 80 of beam 26 is considered. The point 80 is movable along direction 30. The beam 26 is made from light that has an electric field vector 82 only along axis 52. There is no component along axis 54. This is called perfectly linear polarized light, and is polarized in the direction of axis 52. While it appears the same to the human eye as unpolarized light, it has very useful properties, which makes polarizers desirable.
As such, a polarizer is a device or an arrangement that receives randomly polarized light, and permits to exit only linearly polarized light. Moreover, a polarization converter is a term in the art for a device that either rotates the polarization of received light, or converts randomly polarized light into linearly polarized light. In other words, the term polarization converter has come to be used also for a polarizer.
A useful prior polarizer is now described referring to FIG. 4. A polarizing beam splitting (PBS) cube 110 is transparent, and has a hypotenuse surface 112. The cube 110 receives an incident beam of light 116, traveling along an incident direction 117. The cube 110 partially transmits a beam of light 118 along a transmission direction, which is typically identical to the incident direction 117. The PBS cube 110 also partially reflects a beam of light 120 along a reflected direction 122. Preferably the hypotenuse surface 112 is located at a 45.degree. angle from the incident direction 117, in which case reflected direction 122 is at right angles from the incident direction 117.
The incident beam of light 116 is regarded as unpolarized, although that is not necessary. Specifically, one of its electric field vectors I.sub.P is in the same plane as the drawing. Vector I.sub.P is shown as an arrow, and corresponds to the P direction. The other electric field vector I.sub.S is perpendicular to the plane of the drawing, and thus also perpendicular to the paper. Vector I.sub.S is a shown as a circled dot, and corresponds to the S direction.
The transmitted beam of light 118, that exits undeflected from the hypotenuse surface 112 of the cube 110, has a transmission component T.sub.P in the P direction, and a transmission component T.sub.S in the S direction. Similarly, the beam of light 120 that is reflected by the hypotenuse surface 112 has a reflection component R.sub.P in the P direction, and a reflection component R.sub.S in the S direction.
The polarizing beam splitting cube 110 has a very useful property, which is why it is used for making polarizers. Theoretically, it splits the incident beam 116 into a P-polarized transmitted beam, and an S-polarized reflected beam. As such, the polarizing beam splitting cube 110 theoretically separates the incident light into two beams of equal intensity, each polarized only in its own direction.
In practice, the actual polarizing beam splitting cube 110 typically deviates from the above described theoretical performance, but not much. The transmission component T.sub.S is small. For example, if the illuminating beam is a f/2.5 white light, then T.sub.S =0.005.times.I.sub.S. As such, the transmitted beam 118 is light very highly polarized along that the P direction, with a small component in the S direction. Furthermore, the reflection component R.sub.P is relatively small, too. For example, for the same kind of illuminating light, R.sub.P =0.08.times.I.sub.P. As such, the reflected beam 120 is light mostly polarized along the S direction, with the diminished component in the P direction. In each case there is a dominant polarization component as prescribed by theory, but also a minor polarization component.
There are two criteria for gauging the performance of a polarizer. One criterion is how well the undesirable component has been extinguished. The other criterion is how much light intensity of the desirable polarization component is permitted to go through. Like all other real life optical devices, the PBS cube 110 introduces losses, too. For the same kind of illuminating light, T.sub.P =0.92.times.I.sub.P (a loss of 8%), and R.sub.S =0.995.times.I.sub.S (a small loss of 0.5%). A problem with using a PBS cube is that all the light of beam 120 is wasted.
A polarizer 140 in the prior art is now described with reference to FIG. 5. The components of the polarizer 140 are shown separated from each other, i.e. not contacting each other, but that is only for purposes of illustration. It will be appreciated that the inclusion of the secondary cubes rescues a lot of the light that would have been otherwise wasted as beam 120 of FIG. 4.
The polarizer 140 is made from stack of PBS cubes, with their hypotenuse surfaces parallel. Only six cubes 142, 143, 144, 145, 146, 147 are shown. The front surfaces of odd-numbered PBS cubes 143, 145, 147, are respectively obstructed by opaque shields 153, 155, 157. The even numbered PBS cubes 142, 144, 146 have at their rear faces half-wave retarders 162, 164, 166 respectively. The whole stack has a polarizing filter 170 at the exit, to ensure that the undesirable polarization component has been extinguished.
Incident light 176 impinges upon the polarizer 140 along a direction 177, which is shown as many parallel lines. A transmitted beam 178 emerges after the polarizing filter 170, polarized in the desirable direction. A reflected beam 180, polarized substantially only in the undesirable direction, emerges from the side along a side direction 182 and is discarded.
It should be noted that the emerging beam 178 emerges from the entire face of polarizing filter 170. That is notwithstanding the fact that, due to the opaque shields, the incident light enters the polarizer 140 from the front faces of only half of the PBS cubes, namely only from PBS cubes 142, 144, 146. The operation of polarizer 140 is now described with reference to FIG. 6.
Referring now to FIG. 6, the operation of a pair of PBS cubes 144, 145 of the polarizer 140 of FIG. 5 is described. It will be appreciated that the polarizer 140 operates as an aggregation of many such pairs.
The incident beam 176 impinges only upon cube 144 along incident direction 177. Any portion of the incident beam 176 that would ordinarily impinge upon cube 145 is blocked by the opaque shield 155.
Cube 144 acts as a separator of the two beams, each with its own polarization, as described above in connection with FIG. 4. However, a novel convention is employed in FIGS. 6 and 7 of this document, to denote which one is the dominant component, and which one is the minor component, which arises due to the fact that PBS cube 145 does not behave ideally. The novel convention is that, while the dominant component is drawn on the line representing the direction of travel of light, the minor component is drawn outside that line, but still close to the dominant component.
Cube 144 transmits a portion of the light along the incident direction 177, which emerges as beam 188. Right after cube 144, beam 188 has a dominant polarization component 192 in the P direction, and a minor polarization component 194 in the S direction. The half wave retarder 164 rotates these polarization components. As such, reaching the polarizing filter 170 is a dominant polarization component 202 in the S direction (rotated from 192), and a minor polarization component 204 in the P direction (rotated from 194).
The polarizing filter 170 permits only light with polarization similar to that of polarization component 202 to go through. As such, beam 188 is made only by component 212, polarized in the S direction. The polarizing filter 170 thus extinguishes component 204, leaving only highly polarized light.
A PBS cube 144 also reflects a portion of the light along reflected direction 182. That light includes a dominant polarization component 222 in the S direction, and a minor polarization component 224 in the P direction. The reflected light is reflected again on the hypotenuse surface of cube 145, thus traveling along a new direction 226. The new direction 226 is typically made parallel to the incident direction 177. This is easily accomplished if the cubes are identical.
Light traveling along new direction 226 has a dominant polarization component 232 in the S direction (from component 222), and a minor polarization component 234 in the P direction (from component 224). Similarly as above, a beam 238 emerges after filter 170. The filter has extinguished component 234, and thus the beam 238 includes only a component 242 (from 232).
The polarizing filter 170 introduces a lot of losses, but is regarded in the prior art as necessary. The PBS cube 145 does not reflect all the light it receives. Some goes through it as beam 250. Similarly, a PBS cube 144 receives a beam 260. As they travel along direction 182, the beams 250 and 260 reach the subsequent hypotenuse surfaces, and reflect some light in the direction of the transmitted beams. The polarizing filter 170 attenuates that portion of the light that is in the undesirable polarization.